Search

The unification of the electromagnetic and weak forces is a cornerstone of the standard theory of elementary particles and fundamental interactions. At the Large Hadron Collider the processes of pair production via fusion of two exchanged photons provide a unique laboratory both for testing the standard theory and for search of new phenomena in high-energy physics. In this thesis such a two-photon exclusive pair production in pp collisions has been studied experimentally, at two centre of mass energies using the data collected with the CMS experiment during LHC's Run-1. Thanks to large, effective photon fluxes and the outstanding performance of the CMS apparatus clean two-photon signal samples could be extracted. The novel track-based exclusivity selection was instrumental for making successful measurements in an extremely demanding LHC environment. In particular, the "reference" two-photon production of lepton pairs has been measured and investigated in detail, including extended phenomenological studies. A new formalism has been implemented in Monte Carlo simulations using the so-called $k_T$ factorisation scheme. Finally, the two-photon production of weak boson pairs has been studied through the analysis of the data collected with CMS at 7 and 8 TeV. Beside the first observation ever made experimentally of the $\gamma\gamma\rightarrow WW$ process, the measured production rate has been used to extract the best world limits on anomalous behaviour of the $\gamma\gamma WW$ as well as the $\gamma\gamma ZZ$ quartic gauge boson couplings. In addition, the Precision Proton Spectrometer project has been also portrayed - the PPS detectors will allow to measure forward scattered protons, and hence to tag two-photon processes. In particular, one of the possible, ultrafast timing detectors to be installed in the spectrometer is characterised: the GasToF Cerenkov time of flight detector.

My PhD research focuses on exotic Higgs physics. The Higgs boson is the second heaviest observed particle and the latest one to have been discovered. Its first observation at the Large Hadron Collider five years ago was welcomed as great news for many physicists. It was the final confirmation of the Brout-Englert-Higgs mechanism explaining the mass of the other elementary particles that was established more than 50 years ago. However few other physicists were quite disappointed by this discovery. The reason is simple: up to now everything acts as predicted by the main currently accepted theory in particle physics, the Standard Model. Nonetheless even if a lot of its predictions are indeed realized in Nature, there remain unexplained phenomena and small problems inside the theory which call for new physics models. During my PhD I studied several processes in one of these exotic models called the Two-Higgs-Doublet Model in which, instead of having only one Higgs boson, there are five distinct ones. I reviewed three main Higgs boson production modes involving loops at the lowest order in perturbation theory. Given the lack of automated events generation tools for this kind of diagrams in the past, I implemented a reweighting technique to overcome these limitations and to reduce the uncertainty on theoretical predictions by a factor of two. This precision improvement is of vital importance when comparing with experimental data. Finally I put constraints on the top Yukawa couplings in a generic model and I studied its consequences in a simplified Dark Matter model along with other cosmological and collider constraints. (SC - Sciences) -- UCL, 2017

The data from the LHC produced in 2012 at 8 TeV and collected by the CMS experiment are used to probe the scalar sector of the Standard Model (SM) and beyond. Two analyses have been conducted based on an integrated luminosity of 19.5-19.8 fb-1 in the final state with two b jets and two leptons. The decay of the SM Higgs boson to a pair of b quarks is challenging at the LHC. In this thesis we propose the use of a Matrix Element (ME) technique to search for the process Z(ll)h(bb). The ME weights for the background and signal hypotheses are combined thanks to neural networks to search for the presence of the signal. The sensitivity of the analysis does not allow to conclude on the presence or absence of the signal and an upper limit on μ=σ/σSM of 1.6 is derived compatible at 2 standard deviation (s.d.) with the presence of the signal. A search to the well-known ZZ process in the same final state is also conducted giving a result compatible from the expectation from other CMS measurements. The impact of the pile-up on the results of this analysis is also discussed. Several observations are still unexplained by the SM. In this context a search in the frame of two-Higgs-doublets models (2HDMs) is presented. Such models can arise in several beyond standard model theories. The H/A → Z(ll)A/H(bb) processes have been chosen to potentially discover two new resonances. It constitutes the first search at the LHC for these processes. A generic analysis was conducted in order to allow possible recasting of the results in a wide range of models. One interesting excess with a local (global) significance of 2.6 (1.5) s.d. was observed, compatible both in shape and in amplitude with the probed signal with mA = 104 GeV and mH = 270 GeV. Generic limits as a function of mA and mH are derived and used to test a type II 2HDM. Limits for this model is then obtained as a function of mA, mH, cos(β-α) and tanβ. Perspectives to explore low mA in the boosted regime are also discussed. (SC - Sciences) -- UCL, 2016

This talk presents a detailed study of the final state with the Z boson decaying into two leptons, produced in the CMS detector at the LHC. In order to tag this topology, sophisticated b jet tagging algorithms have been used, and the calibration of one of them, the Jet Probability (JP) tagger, is exposed. A study of the tagger degradation at high energy has been done and led to a small gain of performance. This investigation is followed by the search for the associated production of the standard model (SM) Higgs boson with a Z boson and decaying into two b quarks (ZH), using the Matrix Element Method (MEM) and two b-taggers: JP and Combined Secondary Vertex (CSV). The MEM is an advanced tool that produces an event-by-event discriminating variable, called weight. To apply it, several sets of transfer function have been produced. The final results give an observed limit on the ZH production cross section times the H → bb branching ratio of 5.46 when using the CSV tagger and 4.89 when using the JP algorithm: a small excess of data is observed only for CSV. Finally, based on the previous analysis, a model-independent search of new physics has been performed. The goal was to discriminate all the SM processes to categorize the Zbb phase space, with a recursive approach using the MEM weights. Free parameters had to be tuned in order to find the best exclusion limit on the ZH signal strength. For the best configuration, the computed limit was 3.58, close to the one obtained with the dedicated search. To confirm the power of the method, a new physics signal motivated by 2HDM models with a Zbb final state was used leading to a similar conclusion than for the ZH case. (SC - Sciences) -- UCL, 2015

The Standard Model of particle physics has proven to be a very successful theory to describe the fundamental interactions and elementary particles observed in nature. The discovery of a Higgs boson in 2012 by the Atlas and CMS experiments at the LHC has been the ultimate confirmation of the predictions of the theory. However, this model does not have answers to everything at the fundamental level. For example, the inclusion of the gravitational force and the hierarchy problem are among the questions awaiting to be answered. Moreover, it is believed that the fundamental forces could be unified into a single force. Many new theoretical models that address these problems predict the existence of new heavy particles at the TeV scale and even new extra space dimensions. An experimentally and particularly appealing final state to search for these new phenomena at the LHC collider is the di-muon final state. However, no evidence for such new particles has been found so far, indicating that these particles might be heavier and produced at a lower rate than expected. By improving the reconstruction and analysis techniques, the sensitivity of these searches can be enhanced. This thesis presents the results of research performed along both of these lines. First, a new method for mapping the distribution of detector material has been developed and applied to the inner tracker detector of the CMS experiment. The method exploits the scattering of charged particles off the nuclei of the material of a high-energy physics experiment to infer the distribution of the material itself. The method is easily applicable and its accuracy is comparable to that of more traditional methods. The method also proves to be complementary to such traditional methods in that it suffers from different sources of uncertainty. Second, a matrix element based analysis approach has been developed and tested to improve the sensitivity for a discovery of a di-muon resonance. It is found that the analysis method is robust, reliable and advantageous to use in the searches at the LHC. (SC - Sciences) -- UCL, 2015

The start of the LHC in 2009 opened a new era in particle physics. The collisions, produced at an energy and a rate unequalled, allowing to probe energy sectors and rare processes so far inaccessible. In summer 2012 the announcement of the discovery a new boson, presenting the properties of the long-sought Higgs boson was confirming the last prediction of the particle physics theoretical framework called the standard model. This theory, even though highly predictive and never defeated experimentally, is known to have limitations and lacks explanation for several physics observations. The production of a Z boson in association with b jets is a process for which different theoretical predictions from the standard model differ, depending on the treatment of the b jets in the calculation (so-called 4- and 5-flavour schemes). The measurement of the Z plus b jets cross section and the study of its kinematics constitute then a strong QCD test. The Z plus at least two b jets process constitutes the main background for studies of the newly discovered boson, as well as for searches in theories beyond the standard model, presenting the same final state. In this thesis the cross section measurement is done both for Z plus exactly one b jet and Z plus at least two b jets, together giving the result for Z plus at least one b jet. The data used, corresponding to an integrated luminosity of 5 fb−1 , were recorded by the CMS detector from proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. The event selection is led by the care to optimise the signal over background ratio and to select robust well identified objects. b-tagging techniques are used for the selection of the b jets. A special attention is given to the background estimation, partially data-driven when possible. To yield a cross section at the particle level, all the efficiencies of reconstruction and selection and the detector effects are estimated taking into account the migrations in terms of b jet multiplicity. The results are compared with several theoretical predictions coming from calculations at leading order or next-to-leading order and in the two schemes available. Important kinematic variables shown at the reconstructed level are also deconvoluted at the generator level. For this, two different unfolding techniques are explained and used: the Singular Value Decomposition and the Bayesian unfolding. The analysis selection was implemented in the RIVET framework, so that the differential cross sections can be compared to any generator. (SC - Sciences) -- UCL, 2015

This thesis describes a search for production of a Higgs boson in association with a single top quark, conducted in the CMS experiment at the CERN Large Hadron Collider (LHC). This process offers a unique way to study certain aspects of the interaction between the Higgs boson and the top quark. The search targets the anomalous case of a negative top quark Yukawa coupling constant, which leads to a significant enhancement of the tH production cross section with respect to the expectation in the standard model. The study exploits data collected in proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding an accumulated luminosity of about 20/fb. In the considered final state the top quark decays producing a muon or an electron, and the Higgs boson decays into a pair of b quarks. The event selection takes advantage of multiple b-quark jets as well as a jet in the forward region in pseudorapidity. The selected sample of events is largely dominated by the top quark pair production, and artificial neural networks are applied in order to identify signal-like events. A special attention is given to construction of input variables in the multijet final state. Observed data are found to be compatible with the background-only expectation, and an upper limit on the tH production cross section is set. The observed limit at the 95% confidence level is 1.8 pb, which exceeds the expected cross section of the anomalous tH production by a factor of 7.6. Data that will be collected by the LHC in the years to come will allow to rule out (or discover) the considered anomalous tH production. (SC - Sciences) -- UCL, 2015

Almost half a century after its prediction, the Higgs boson has been discovered in 2012 by the ATLAS and the CMS collaborations with the data accumulated during the first run of the LHC. While the existence of the electroweak symmetry breaking mechanism is now a piece of evidence, the scalar particle content could not be limited to the Higgs boson. A simple extension of the scalar sector, the 2HDM, predicts the existence of five scalar bosons and provides a very rich phenomenology and therefore new channels to test the standard model at the LHC. In particular, the first searches for the production of a heavy scalar decaying to a pseudoscalar and a Z boson have been carried out in this thesis in the final state with two leptons and two b-jets. No evidence of new physics has been found with the data collected in 2012 and 2015, we have therefore partially excluded the parameter space in regions which are unreachable using only the standard model Higgs boson properties. Having in mind the large variety of 2HDM and more complex models that could involve such processes, the analysis strategy has been chosen to be easily re-casted or reinterpreted. Alongside this, we have provided the community with a modular parametric detector simulation called Delphes. The program has shown to realistically reproduce the performance of the CMS detector. It is currently widely used by both the experimental collaborations and the phenomenologists. (SC - Sciences) -- UCL, 2017